1. Field Of The Invention
The present invention relates to a pulse width modulation type inverter for delivering an A.C. current at variable voltage and variable frequency. More particularly, the present invention is concerned with the control of the pulse width modulation signal (referred to as a "PWM" signal, hereinafter) in an inverter using a high carrier frequency.
2. Description Of The Prior Art
FIG. 10 is an illustration of the arrangement of a conventional PWM inverter. Referring to the drawings, the known PWM inverter includes a D.C. power supply 100 and an inversion converter 200 composed of a controllable element and a pair of inverse parallel-connected diodes. The inversion converter is capable of converting a D.C. current into an A.C. current of variable voltage and variable frequency, which is useable to drive an electric motor 300, or similar device. A reference voltage generator 400 generates a reference voltage waveform, which is used as a reference for the output frequency and output voltage. In practice, the waveform of the reference voltage has the optimum shape for operating the motor, but such a shaped waveform cannot be obtained with adequate power where commercial electrical power is not available. Thus, a carrier waveform having adequate power is mixed with the reference wave and results in a pulsed waveform having the same area as the reference voltage wave, adequate to drive the motor or like device. The reference waveform is modified in response to the input from an output frequency setting device 900 that is setable by a user. A suitable carrier generator 500 will form a carrier signal, for example, a triangular waveform, at a frequency f.sub.c. A PWM circuit 600 is operative in response to the signals from the reference voltage generator 400 and the carrier generator 500 to produce a control signal for the controllable element of the inversion converter 200. A driving circuit 700 will drive the controllable element of the inversion converter 200 in response to the signal from the PWM circuit 600 and an output frequency setting device 900.
The operation of this known inverter will be describe with reference to FIG. 11, which illustrates several waveforms that occur during a typical PWM operation. It should be noted that the illustration is in regard to only one phase, specifically the "U phase", of the three phases U, V and W relevant to the actual inverter operation in producing 3-phase A.C. power for operating a motor or like device. Referring to FIG. 11(a), the triangular waveform output of the carrier generator 500 and the sinusoidal waveform output by the reference voltage generator 400 are shown superimposed in the same time frame. The superimposed waveforms illustrate a comparison between the reference voltage, which is used as a reference for the output voltage and the output frequency of the inverter, and the signal for modulating the reference voltage, e.g., the triangular carrier waveform. Waveforms in FIG. 11(b) are generated on the basis of the points of the waveforms of FIG. 11(a) where the reference voltage waveform and the carrier cross. One of the waveforms in FIG. 11(b) is a PWM signal U.sub.po. This signal is generated for the upper side of the U phase and the signal is ON in the period in which the reference voltage is higher than the carrier voltage and OFF in the period in which the reference voltage is lower than that of the carrier. The other waveform in FIG. 11(b) is a PWM signal U.sub.NO. This signal is generated for the lower side of the U phase and is obtained as the inversion oi the signal U.sub.PO. The controllable element is actually driven by the PWM signals Up and Un, as seen in FIG. 11(c), which signals are formed from waveforms U.sub.PO and U.sub.NO as subjected to a short circuit prevention process which delays the timing of the ON pulse by a time duration T.sub.d. As a result of this delay, the pulse-width modulated output voltage obtained as the U phase output is shown in FIG. 11(d). Similar outputs are obtained for the V and W phases.
Referring again to FIG. 10, the reference voltage waveform shown in FIG. 11(a), which provides a reference for output frequency and output voltage, is delivered by the reference voltage waveform generator 400. The triangular carrier wave shown in FIG. 11(a) is generated by the carrier generator 500. The PWM circuit 600 responds to the waveforms of FIG. 11(a) and generates a PWM signal as shown in FIG. 11(c). The driving circuit 700 amplifies the output of circuit 600 and drives the controllable element of the inversion converter 200 accordingly. Since the driving signal will vary with changes in the reference voltage waveform, an A.C. current may be obtained from the inverter with a variable voltage and a variable frequency.
When an electric motor is driven with this type of PWM waveform, higher harmonics, in the audible range, are generated due to the carrier frequency. These audible signals will increase the level of noise in a work environment. One countermeasure for averting this problem is to increase the carrier frequency to the upper limit of the human audible frequency range (i.e., 15 KHz) or higher. The level of noise will progressively decrease as the carrier frequency is increased. In fact, when the carrier frequency is within the range between 10 KHz and 15 KHz, the noise frequency approaches the upper limit of the audible frequency range and the noise level is lowered. When the carrier frequency is increased beyond 20 Khz the audible frequency range is exceeded so that the higher harmonics are not detectable by human ears. As a result, the noise level is reduced almost to the level produced when the load is driven by commercial electrical power.
In order to achieve a high carrier frequency accompanied by low noise, a high-speed switching element such as a power MOSFET, IGBT or the like, operable at a frequency between 10-20 Khz, may be used. The disadvantage of such design is that the switching is inevitably accompanied by a significant amount of power loss. More specifically, the loss P generated by a controllable element accompanied by inverse-parallel diodes, is given by the following formula: ##EQU1##
The "ordinary loss" is the product of the current flowing during the ON time and the voltage drop, while the "switching loss" is the product of voltage and current at the time that the controllable element is turned ON and OFF. In simplified form, as indicated in equation (1), the total ordinary loss P.sub.ON is merely a function of current level (I), while the total switching loss P.sub.SW is a function of current level (I) and the carrier frequency (f.sub.c) which controls the switching of the element.
The switching loss P.sub.SW is increased as the carrier frequency f.sub.c is increased. Also, the ratio of loss P.sub.ON : P.sub.SW is large when the current is near the level of the rated current of the inverter. When inverter operation results in a relatively high switching loss, i.e., high f.sub.c and high current, since proper thermal design of the switching element requires that its junction temperature be maintained below a given level, there is a significant cooling requirement. Thus, operation of the inverter at a high carrier frequency will reduce audible noise, but will require both an increase in the cooling capability and the size of the inverter.
The noise produced poses a serious problem when an inverter-driven electric motor operates at low speed. At low motor speeds, the noise generated by the load driven by the motor is relatively low and the motor noise tends to be dominant. At higher motor speeds, the noise generated by the load driven by the motor is increased so that the higher harmonic sounds produced by the electric motor are not dominant and their effect is not critical. Nevertheless, in the conventional inverter, when the speed of the motor is increased, f.sub.c is maintained high so that the loss is increased due to high switching losses. In order to satisfy the thermal requirements of the switching element, the conventional inverter must be designed to maintain the temperature of the switching element below a maximum allowable temperature. Clearly, when the ambient temperature is low, the inverter can operate safely even when f.sub.c is kept high after an increase in the load. However, when the ambient temperature begins to rise, the most effective parameter available to contain the temperature below the maximum for the switching element is the carrier frequency f.sub.c. If carrier frequency f.sub.c is reduced to reduce the temperature rise due to switching power losses, however, it is difficult to effectively suppress the generation of audible noise.
The present invention is intended to solve the above-described problems of the prior art. Thus, an object of the present invention is to provide an inverter apparatus which can operate with a lowered level of audible noise and which can compensate for increases in loss without requiring the size of the inverter to be increased or special cooling to be applied.
A further object of the present invention is to control the carrier frequency in accordance with the inverter output frequency or the temperature of the controllable elements, or output current, so that the generation of noise is suppressed and the loss of power is reduced.